1. Field of the Invention
The invention concerns the technical field of reinforcement materials adapted to the creation of composite parts. More specifically, the invention concerns a new multiaxial stack for fabricating composite parts, by subsequent injection or infusion of thermosetting resin, and its fabrication process.
2. Description of Related Art
The fabrication of composite parts or goods, that is, containing on one hand one or several reinforcements or fibrous layers, and on the other hand, a primarily thermosetting matrix (“resin”) that may include thermoplastics, can for instance be achieved by a process called “direct” or “LCM” (“Liquid Composite Molding” in English). A direct process is defined by the fact that one or several fiber reinforcements are handled in a “dry” state (that is, without the final matrix), the resin or the matrix being handled separately, for instance by injection into the mold containing the fiber reinforcemnts (“RTM” process, “Resin Transfer Molding” in English), by infusion through the thickness of the fiber reinforcemnts (“LRI” process, “Liquid Resin Infusion” in English or “RFI” process, “Resin Film Infusion” in English), or else by manual coating/impregnation with a roller or brush on each of the unit layers of fibrous reinforcement, applied successively on the mold.
For the RTM, LRI or RFI processes, it is generally first necessary to build a fibrous preform of the mold of the desired finished product, then to impregnate this preform with a resin. The resin is injected or infused by a pressure differential at temperature, then once all the amount of necessary resin is contained in the preform, the assembly is brought to a higher temperature to complete the polymerization/reticulation cycle and thus activate its hardening.
In these sectors, a large number of preforms are fabricated from a reinforcement material, primarily carbon fibers, notably of the unidirectional type. The resin that is subsequently associated, notably by injection or infusion, with the unidirectional reinforcement layers during the creation of the part, can be a thermosetting resin, of an epoxy type for instance. In order to allow a correct flow through a preform composed of a stack of different layers of carbon fibers, this resin is most often very fluid. The major inconvenience of this type of resin is its fragility after polymerization/reticulation, which results in poor resistance to impact for the fabricated composite parts.
In order to solve this problem, the documents of previous art suggested that the unidirectional layers of the carbon fibers be associated with a web of thermoplastic fibers. Solutions such as these are notably described in the patent applications or patents EP 1125728, U.S. Pat. No. 628,016, WO 2007/015706, WO 2006/121961 and U.S. Pat. No. 6,503,856. The addition of this web makes it possible to improve mechanical properties in the compression after impact (CAI) test commonly used to characterize the impact resistance of the structures.
Furthermore, for the fabrication of composite parts, multiaxial stacks are very often used comprising unidirectional layers extending in different directions. Most often, these unidirectional layers are attached to each other by stitching or knitting.
Patent application EP 1473132 in turn, describes a stack of multiaxial unidirectional layers associated with webs, the connection between the unidirectional layers of this stack being achieved by means of heated rollers which provide a continuous weld.
Document EP 1348791 describes a combination of unidirectional carbon layers in which the strands are oriented parallel to each other, the connection between the layers being obtained by means of thermoplastic bonding filaments which are aligned and spaced apart from each other.